General map of Ambrym showing the main volcanic features (caldera, cones,
maars, and fissures). Ambrym is the most voluminous active volcano in
Vanuatu. Marum and Benbow are post-caldera cones. The caldera is similar
in size to calderas associated with large Plinian eruptions. From Robin and
others (1993).

Ambrym Island is a large shield volcano with a caldera. The caldera
formed in about 50 A.D. with an eruption of VEI 6. It is also a very
active volcano with 48 eruptions since 1774. Most of these eruptions are
at cones inside of a caldera (5.5 by 7.5 miles; 9 by 12 km).The eruptions
are explosive and usually from a central vent, There have been a few
flank eruptions. Fifteen erupts produced lava flows. Ten
eruptions involved a lava lake. There were fatalities in two eruptions at
Ambrym. In 1894, six people were killed by volcanic bombs and four
people were overtaken by lava flows. In 1913, 21 people were killed
during an explosive eruption. Acid rainfall in 1979 burned some of the
inhabitants. Local water supplies were contaminated and had a pH of 5.2-5.5.

View of the ash plain from a ridge on Benbow. The caldera rim is in the
distance on the right. The caldera is about 7.5 miles (12 km) in
diameter. Photograph courtesy of Mike Lyvers.